The present invention relates to a magnetoresistive disc drive head for high frequency and high data rate applications, and in particular to a magnetoresistive disc drive head having the capability of separating direct magnetic field effects and an inductive time rate of change effect resulting from a magnetic field and canceling the undesired inductive time rate of change effect.
Magnetoresistive (MR) sensors arc used in magnetic storage systems to detect magnetically encoded information. A time dependent magnetic field from a magnetic storage medium or disc directly modulates the resistivity of the MR sensor. The change in resistance of the MR sensor can be detected by passing a sense current through the MR sensor and measuring the voltage across the MR sensor. The resulting signal can be used to recover information from a magnetic storage medium or disc.
Practical MR sensors are typically formed using ferromagnetic metal alloys because of their high magnetic permeabilty. A ferromagnetic metal alloy is deposited in a thin film upon an electrically insulated substrate or wafer. Changing magnetic fields originating from the magnetic storage medium produce a change in the magnetization direction of the MR sensor and thereby change the resistance of the sensor. This phenomenon is called an MR effect.
MR sensors have a maximum signal-to-noise ratio when the active region of the sensor has no movable magnetic domain boundaries or no domain boundaries. In other words, the active sense area of the MR sensor should be a single domain. The presence of domain boundaries in the sensor active area that move when a field is applied gives rise to Barkhausen noise, a phenomenon caused by the irreversible motion of a magnetic domain in the presence of an applied magnetic field. Barkhausen noise cannot occur if no domain boundaries exist. Typically, a single magnetic domain MR sensor is achieved by either utilizing geometry, or via boundary control stabilization or inherent longitudinal magnetic fields or any combination thereof.
During a read operation, an MR sensor transduces the data field of a medium directly by virtue of an MR effect and produces an MR voltage signal. However, the MR sensor also couples an ideally 90.degree. out of phase voltage signal due to the inductive pickup from the contact loop configuration of the sensor current path providing current to the MR sensor (neglecting capacitance). The out of phase signal is undesired because it adds a coherent signal that is phase shifted away from the real MR signal attempting to be detected through use of the MR effect. Therefore, the MR sensor is detecting two different signals from a single medium. First, the MR sensor detects the MR signal representing the magnetic field directly. Second, the extended MR sensor contact structure detects the inductive pick-up signal from the time rate of change of magnetic flux of the medium linking by the single-loop contact configuration of the current path. The purpose of the MR sensor is to detect the MR signal representative of the magnetic field of the medium directly, and not the inductive pick-up signal from the time rate of change of the magnetic flux.
Inductive voltage signal detection is not normally associated with MR sensor operations due to the fact that most applications utilizing MR sensors have a relatively low disc velocity. The single-loop contact configuration of a conventional MR sensor current path (wherein the single loop is encompassed by the MR sensor, bond pads, and the electrical contacts connection the MR sensor to the bond pads) will not produce significant inductive time rate of change signals from a disc since its inductive output is directly proportional to disc velocity, which itself is relatively small.
In MR sensor operations having a relatively small head disc velocity, the inductive pickup signal induced by the single-loop contact configuration of a conventional MR sensor current path is often more than 40 db down in magnitude from the MR signal itself. As a result, induced noise does not substantially affect the MR signal due to its relatively small magnitude as compared to the MR signal. However, as a relative head disc velocity increases, the inductive pick-up signal transduced by the single-loop contact configuration of the MR sensor current path can be only 10-20 db in magnitude below the MR signal. This induced signal can represent a serious signal-to-noise ratio problem.
In high performance disc drive applications having large relative head disc velocity, the single loop contact configuration of a conventional MR sensor can transduce an inductive pick-up signal which is 90.degree. out of phase with a desired MR signal and with a magnitude that causes error by peak shifting the information away from the desired timing windows of the information channel. The inductive pick-up signal results in head disc channel performance errors.
Thus, there is a need for a disc drive head for reading an MR voltage signal representing the magnetic flux from a magnetic storage medium directly while minimizing any induced inductive pick-up signal representing time rate of change of magnetic flux from the medium.